In operation of a gas turbine engine, air at atmospheric pressure is initially compressed by a compressor and delivered to a combustion stage. In the combustion stage, heat is added to the air leaving the compressor by adding fuel to the air and burning it. The gas flow resulting from combustion of fuel in the combustion stage then expands through a turbine, delivering up some of its energy to drive the turbine and produce mechanical power.
The gases within the combustor typically range from between 2000 degrees to at least 2500 degrees Fahrenheit. Since the efficiency and work output of the turbine engine are related to the entry temperature of the incoming gases, there is a trend in gas turbine engine technology to increase the gas temperature. A consequence of this is that the materials of which the combustor, blades and vanes are made assume ever-increasing importance with a view to resisting the effects of elevated temperature.
Historically, combustors have been made of metals such as high temperature steels and, more recently, nickel alloys, and it has been found necessary to provide internal cooling passages in order to prevent melting. It has been found that ceramic coatings can enhance the heat resistance of the turbine components. In specialized applications, nozzle guide vanes and blades are being made entirely of ceramic, thus, imparting resistance to even higher gas entry temperatures and requiring higher temperatures within the combustor.
However, if the combustor is made of ceramic, which has a different chemical composition, physical property and coefficient of thermal expansion to that of a metal supporting structure, then undesirable stresses, a portion of which are thermal stresses, will be set up between the combustor and its supports when the engine is operating. It is felt that such undesirable thermal stresses cannot adequately be controlled by cooling.
Furthermore, conventional assembly techniques and methods will require alternative designs, processes and assembly techniques. The structural components of the combustor and the assembly of the combustor within the gas turbine engine will need to be rethought.
Historically, using metallic components, a combustor design has used a multipiece design of segments one overlaps another. The segments are rigidity secured one to another by rivets, bolts and/or welding. Or as an alternative, the combustor has been formed from a single piece. With a ceramic combustor, the integrity of the material and the construction thereof can drastically increase cost and result in premature failure due to flaws in the surface or of the part itself. The larger the physical size of the ceramic shape the lesser the likelihood of producing a component having structural integrity. The sliding friction between the ceramic combustor and the supporting structure can create a contact tensile stress on the ceramic that degrades the surface. If this degradation in the surface of the ceramic occurs in a tensile stress zone of the combustor the surface flaw generated can result in catastrophic failure.
The present invention is directed to overcome one or more of the problems as set forth above.